1. Field of the Invention
Aspects of the present invention relate to a secondary battery, and more particularly, to a lithium secondary battery having an improved structure of a can or a case to keep the outer thickness of the secondary battery uniform.
2. Description of the Related Art
As the information society has been rapidly developing, users' need for smaller, lighter and multi-functional devices has increased. Thus, portable electronic devices (such as PDAs, GPSs, camcorders, mobile phones, etc.) employ batteries as their main power source. To make devices smaller and lighter, secondary batteries, which have a high energy density and are rechargeable, are adopted for most battery power source needs.
In particular, since lithium is light, lithium secondary batteries have a higher energy density per unit weight and a faster charging ability than conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc., and thus are widely used.
A secondary battery is typically formed by putting an electrode assembly including a positive electrode, a negative electrode and a separator into a can, which is typically formed of aluminum or an aluminum alloy, covering the can with a cap assembly, injecting an electrolyte into the can, and sealing the can. The term “bare cell” is used herein to refer to the sealed can containing the electrode assembly and electrolyte and including the cap assembly, before additional components such as safety devices, additional outer cases, labels, etc., are added.
While the can may be formed of a material such as iron, aluminum or an aluminum alloy provides advantages as a material forming a can in that the battery can be lightweight and not decay over a long period of use under high voltage.
Batteries have a possibility to output high energy as a power source. A secondary battery accumulates high energy in itself in a charged state and also accumulates energy provided from another energy source while charging. When defects such as an inner short-circuit occur in the secondary battery during charging or in a charged state, the energy accumulated in the battery may be emitted in a short period time, thereby causing safety problems such as ignition, explosion, etc.
Lithium secondary batteries may easily ignite or explode in the event of a battery error because of the high activity of lithium itself. Lithium ion batteries are more stable than batteries employing metallic lithium, since only ionic lithium, not metallic lithium, remains therein. Nevertheless, materials such as the negative electrode or nonaqueous solvent used in the battery still have a risk of ignition or explosion in the event of a battery error, because they are flammable.
Accordingly, the secondary battery may include several safety devices to prevent ignition or explosion caused by battery error in a charged state or during a charging step. The safety devices are connected with positive and negative terminals of the bare cell by a conductive structure commonly called a lead plate.
These safety devices prevent hazards (i.e., explosion, ignition, etc.) of a battery by blocking current when the voltage of the battery is abruptly increased due to an increase in temperature or excessive charging/discharging. The safety device, which includes a protection circuit for sensing abnormal current or voltage and blocking current flow, a positive temperature coefficient (PTC) device operated by overheating due to the abnormal current, or a biometal, is connected to the bare cell.
The secondary batteries having a circuit part containing a safety device, etc., connected to a bare cell with a molded resin are broadly classified into two types depending on an outer shape in the final application. The two types are a hard pack battery and an inner pack battery. In the hard pack battery, a terminal (outer terminal) electrically connected to an external device is exposed and a housing formed of plastic resin is integrated with an outer surface of the battery. In the inner pack battery, an outer terminal is exposed and a housing formed of plastic resin is not separately molded on an outer surface of the battery but a label is wrapped.
The inner pack battery can be used irrespective of the shape and structure of the external device, and can be made in slim and various designs, thereby being widely applied to various products such a slim mobile phone and is rapidly standardized.
Meanwhile, with the trend to miniaturization of electronic devices employing secondary batteries, the need for smaller, and more specifically, thinner secondary batteries which provide energy to an electronic device is increasing.
In general, it is desirable for the secondary battery to be formed to a thickness of 4.7 mm±0.1 mm, which may be changed a little according to the product in which the battery is to be used. Since a battery becomes thicker in its middle part during use of the battery because of a swelling phenomenon, the middle part should have a thickness of 4.7 mm or less in an un-swelled state in order to have a swelled thickness of 4.7 mm.
Accordingly, when a film-type label is wrapped on the outer surface of the bare cell, the label overlaps itself at a certain part of the bare cell for stronger attachment, and thus a region in which the label overlaps itself (i.e., a label overlapping portion) is formed.
That is, the battery is thicker because of the thickness of the label, particularly at the label overlapping portion, which makes thickness management of the secondary battery difficult. The increase in thickness due to the thickness of the label may not be reduced by management, so that another cause of thickness difference, that is, a thickness difference of a can, has to be strictly controlled.